The market of a solid electrolytic capacitor using an electroconductive polymer served as a solid electrolyte has been rapidly enhanced because it has various superior properties including reduced equivalent series resistance (ESR) and improved reliability, compared with conventional solid electrolytic capacitors using manganese dioxide as a solid electrolyte.
The electroconductive polymer is usually prepared by a chemical oxidation polymerization method. For example, as an oxidant and dopant or a transition metal salt of an organic sulfonic acid, such as iron paratoluene sulfonate, is used, while a monomer such as thiophene or derivatives thereof is polymerized. (See JP Laid-open patent publication Nos. JP10-50558 and JP2000-106331.)
However, these methods are useful in mass production, but there is an objective that the transition metal served as an oxidant remains in the electroconductive polymer. In order to remove the transition metal, a washing process is applied, but even if washing it, it is inherently difficult to completely remove the transition metal due to its property. When the transition metal remains in the electroconductive polymer, the transition metal affects the electroconductive polymer or a solid electrolytic capacitor using it as a solid electrolyte. Therefore, it has been required to get rid of the affects on the solid electrolytic capacitor to further improve the stability of the electroconductive polymer to assure the long-term reliability of the solid electrolytic capacitor. To the end, an oxidant other than a transition metal salt has been proposed, such as a peroxide as an alternative oxidant. However, a monomer such as thiophene or a derivative thereof is used, the reactivity becomes significantly low and the electric conductivity of the electroconductive polymer as obtained is much lower than those when using a transition metal salt